The present invention relates to a semiconductor device and more specifically to a high-density, high-speed dynamic semiconductor memory device.
Conventional semiconductor memory devices include dynamic semiconductor memory devices (hereinafter referred to as DRAMs: Dynamic Random Access Memories) in which each memory cell is composed of one cell capacitor and one cell transistor. The DRAMs are adapted for high packing density because each memory cell takes up a small amount of area. In recent years, therefore, the development of products has been pushed forward in various places to increase the packing density and operating speed of the DRAMS.
In order to increase the packing density, it is required to reduce the thickness of the gate oxide film of each cell transistor and thereby scale down the dimensions of it. However, the thinner the gate oxide film, the lower the supply voltage to the memory cell array has to be set, since the withstand voltage is lowered. On the other hand, the lower the supply voltage to the memory cell array, the lower the signal level for sensing data stored in each memory cell becomes. As the supply voltage to the memory cell array is lowered, the threshold voltage V.sub.thn of the N-channel MOS transistors of each flip-flop (F/F) that discriminates a signal voltage in the initial stage of a sense operation has to be lowered. However, it is not easy to lower the threshold voltage V.sub.thn according to scaling rules.
The sense margin of sense amplifiers depends on the threshold voltage V.sub.thn. Thus, when the magnitude of the threshold voltage V.sub.thn is large and the signal level to be detected is low, the normal sense operation becomes difficult to perform. Difficulties are therefore involved in adopting conventional sense amplifiers.
The S/N ratio of the conventional sense amplifiers depends on the transition characteristic (driving speed K) of the common source voltage of N-channel MOS transistors in each F/F because of unbalance of electric parameters of a pair of bit lines and a sense amplifier. With the conventional sense amplifiers, therefore, the lower the driving speed K with which the precharged voltage discharges down to ground voltage, the higher the sensitivity becomes. Thus, the conventional sense amplifiers have a problem that, as semiconductor memory devices advance in fine device structure and speed, it becomes more and more difficult to sense the signal level of each memory cell.